The invention relates to steganography, and in particular relates to hiding data in media objects such as images, video and audio.
Steganography refers to a process of hiding information in other information. One form of steganography is digital watermarking. Digital watermarking is a process for modifying media content to embed a machine-readable code into the data content. The data may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media such as images, audio signals, and video signals. However, it may also be applied to other types of data, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark by altering data samples of the media content in the spatial, temporal or some other transform domain (e.g., Fourier, Discrete Cosine, Wavelet Transform domains). The reading component analyzes target content to detect whether a watermark is present. In applications where the watermark encodes information (e.g., a message), the reader extracts this information from the detected watermark.
The present assignee""s work in steganography, data hiding and watermarking is reflected in U.S. Pat. No. 5,862,260; in application Ser. No. 09/503,881 and now U.S. Pat. No. 6,408,082; and in published specifications WO 9953428 and WO0007356 (corresponding to U.S. Pat. Nos. 6,449,377 and 6,345,104). A great many other approaches are familiar to those skilled in the art. The artisan is presumed to be familiar with the full range of literature about steganography, data hiding and watermarking.
The invention provides a wavelet based, feature modulation watermark system and methods. In addition, it provides a variety of fragile and semi-fragile watermark systems and methods that may be used to detect alteration or copying of watermarked signals or physical objects.
One aspect of the invention is a watermark embedder. The watermark embedder decomposes a media signal from its perceptual domain to subbands and embeds a message signal in the edge information of neighboring coefficients of those subbands.
Another aspect of the invention is a watermark decoder. The watermark decoder decomposes the watermarked signal into subbands and demodulates the message signal from the edge information of neighboring coefficients.
This document describes a variety of implementation details and applications of watermark embedders and decoders. In addition to the message signal, for example, the embedder may also encode an orientation signal to synchronize the decoder with the embedded signal in a distorted version of the watermarked signal.
The watermark system may be used in a variety of applications, including robustly carrying metadata or links to metadata, and to detect alterations of the watermarked signal, such as alterations due to printing, scanning, compression, etc. The watermark may be used to detect alteration by evaluating changes to instances of a message signal decoded from a host media signal. In one application for example, a watermark decoder evaluates changes in a watermark message signal relative to a reference message signal. The reference may be the original message signal embedded in the host media signal, instances of the message signal decoded from different parts of the media signal (e.g., different subbands), or a combination of both. In addition, the watermark may be used to carry authentication information, such as a hash of the host signal, that is used to detect alteration. The watermark decoder compares the hash extracted from the watermark with a hash re-computed from the host signal to detect alteration. The decoder may also use the extent and nature of the detected alteration to distinguish among different types of alterations, such as those due to printing, scanning, compression, etc.
Further features will become apparent with reference to the following detailed description and accompanying drawings.